Polymer and positive resist composition

ABSTRACT

Provided are a polymer that can be favorably used as a positive resist having a low film reduction rate in a state of low irradiation with ionizing radiation or the like and a positive resist composition that can favorably form a high-resolution pattern. The polymer includes an α-methylstyrene unit and a methyl α-chloroacrylate unit, and the proportion of components having a molecular weight of less than 6,000 in the polymer is no greater than 0.5%. The positive resist composition contains the aforementioned polymer and a solvent.

TECHNICAL FIELD

The present disclosure relates to a polymer and a positive resistcomposition, and in particular relates to a polymer that is suitable foruse as a positive resist and a positive resist composition that containsthis polymer.

BACKGROUND

Polymers that display increased solubility in a developer afterundergoing main chain scission through irradiation with ionizingradiation, such as an electron beam, or short-wavelength light, such asultraviolet light, are conventionally used as main chain scission-typepositive resists in fields such as semiconductor production.(Hereinafter, the term “ionizing radiation or the like” is used to refercollectively to ionizing radiation and short-wavelength light.)

PTL 1 discloses one example of a main chain scission-type positiveresist having high sensitivity. The disclosed positive resist is formedfrom an α-methylstyrene-methyl α-chloroacrylate copolymer including anα-methylstyrene unit and a methyl α-chloroacrylate unit.

CITATION LIST Patent Literature

PTL 1: JP H8-3636 B

SUMMARY Technical Problem

In order to refine and increase the resolution of a pattern obtainedusing a main chain scission-type positive resist, there is demand for aresist that enables clear division, to as great an extent as possible,between regions that undergo main chain scission through irradiationwith ionizing radiation or the like and then dissolve in a developer,and remaining regions that do not dissolve in the developer. From aviewpoint of forming a finer pattern, it is preferable to use a resistthat has low solubility in a developer when exposed to a lower dose ofirradiation than the dose that resist regions to be dissolved need to beexposed to so as to become sufficiently soluble. This is in order toreduce top-collapse of resist regions that are to be left without beingdissolved. Accordingly, the resist is required to have a low filmreduction rate in a state of low irradiation with ionizing radiation orthe like (film reduction rate under low irradiation).

However, the positive resist formed from the α-methylstyrene-methylα-chloroacrylate copolymer described in PTL 1 suffers from a problem ofexcessive film reduction in a state of low irradiation with ionizingradiation or the like. Accordingly, there is room for improvement overthe positive resist formed from the α-methylstyrene-methylα-chloroacrylate copolymer described in PTL 1 in terms of furtherreducing the film reduction rate in a state of low irradiation withionizing radiation or the like.

One objective of the present disclosure is to provide a polymer that canbe suitably used as a positive resist having a low film reduction ratein a state of low irradiation with ionizing radiation or the like.

Another objective of the present disclosure is to provide a positiveresist composition that can favorably form a high-resolution pattern.

Solution to Problem

The inventor conducted diligent investigation to achieve the objectivesset forth above. Through this investigation, the inventor discoveredthat an α-methylstyrene-methyl α-chloroacrylate copolymer in which theproportion of components having a molecular weight of less than 6,000 isno greater than a specific value can be favorably used as a positiveresist having a low film reduction rate in a state of low irradiationwith ionizing radiation or the like. This discovery led to the presentdisclosure.

Specifically, the present disclosure aims to advantageously solve theproblems set forth above by disclosing a polymer including anα-methylstyrene unit and a methyl α-chloroacrylate unit, wherein aproportion of components having a molecular weight of less than 6,000 isno greater than 0.5%. An α-methylstyrene-methyl α-chloroacrylatecopolymer in which the proportion of components having a molecularweight of less than 6,000 is no greater than 0.5% can be favorably usedas a positive resist since the copolymer has a low film reduction rateunder low irradiation during use as a positive resist.

The “proportion of components having a molecular weight of less than6,000” referred to herein can be determined using a chromatogramobtained through gel permeation chromatography by calculating the totalarea (C) of peaks in the chromatogram for components having a molecularweight of less than 6,000 as a proportion (=(C/A)×100%) relative to thetotal area (A) of all peaks in the chromatogram.

In the presently disclosed polymer, a proportion of components having amolecular weight of less than 10,000 is preferably no greater than 0.5%.As a result of the proportion of components having a molecular weight ofless than 10,000 being no greater than 0.5%, the film reduction rateunder low irradiation during use as a positive resist can be furtherreduced.

The “proportion of components having a molecular weight of less than10,000” referred to herein can be determined using a chromatogramobtained through gel permeation chromatography by calculating the totalarea (B) of peaks in the chromatogram for components having a molecularweight of less than 10,000 as a proportion (=(B/A)×100%) relative to thetotal area (A) of all peaks in the chromatogram.

In the presently disclosed polymer, a proportion of components having amolecular weight of greater than 80,000 is preferably at least 15%. As aresult of the proportion of components having a molecular weight ofgreater than 80,000 being at least 15%, the film reduction rate underlow irradiation during use as a positive resist can be further reduced.

The presently disclosed polymer preferably has a weight averagemolecular weight (Mw) of at least 55,000. As a result of the weightaverage molecular weight (Mw) being at least 55,000, the film reductionrate under low irradiation during use as a positive resist can befurther reduced.

The “weight average molecular weight (Mw)” referred to herein can bemeasured by gel permeation chromatography.

Moreover, the present disclosure aims to advantageously solve theproblems set forth above by disclosing a positive resist compositioncontaining any one of the polymers described above and a solvent. As aresult of the above-described polymer being contained as a positiveresist, a high-resolution pattern with low top-collapse can be favorablyformed.

Advantageous Effect

Through the presently disclosed polymer, it is possible to provide apositive resist having a low film reduction rate in a state of lowirradiation with ionizing radiation or the like.

Moreover, through the presently disclosed positive resist composition,it is possible to favorably form a high-resolution pattern.

DETAILED DESCRIPTION

The following provides a detailed description of embodiments of thepresent disclosure.

The presently disclosed polymer can be favorably used as a main chainscission-type positive resist that undergoes main chain scission tolower molecular weight upon irradiation with ionizing radiation, such asan electron beam, or short-wavelength light, such as ultraviolet light.The presently disclosed positive resist composition contains thepresently disclosed polymer as a positive resist.

(Polymer)

The presently disclosed polymer is an α-methylstyrene-methylα-chloroacrylate copolymer that includes an α-methylstyrene unit and amethyl α-chloroacrylate unit, and in which the proportion of componentshaving a molecular weight of less than 6,000 is no greater than 0.5%. Asa result of the presently disclosed polymer including a structural unitderived from methyl α-chloroacrylate having a chloro group (—Cl) at theα-position (i.e., a methyl α-chloroacrylate unit), the presentlydisclosed polymer readily undergoes main chain scission to lowermolecular weight upon irradiation with ionizing radiation or the like(for example, an electron beam, a KrF laser, an ArF laser, or an EUVlaser). Moreover, as a result of the proportion of components having amolecular weight of less than 6,000 in the presently disclosed polymerbeing no greater than 0.5%, the presently disclosed polymer can befavorably used as a main chain scission-type positive resist since thepresently disclosed polymer has a low film reduction rate under lowirradiation during use as a positive resist.

<α-Methylstyrene Unit>

The α-methylstyrene unit is a structural unit derived fromα-methylstyrene. As a result of the presently disclosed polymerincluding the α-methylstyrene unit, the presently disclosed polymerdisplays excellent dry etching resistance during use as a positiveresist due to the protective stability of the benzene ring in theα-methylstyrene unit.

The presently disclosed polymer preferably comprises at least 30 mol %and no greater than 70 mol % of α-methylstyrene units.

<Methyl α-Chloroacrylate Unit>

The methyl α-chloroacrylate unit is a structural unit derived frommethyl α-chloroacrylate. As a result of the presently disclosed polymerincluding the methyl α-chloroacrylate unit, the presently disclosedpolymer readily undergoes main chain scission through chlorine atomdissociation and β-cleavage reaction upon irradiation with ionizingradiation or the like. Accordingly, a positive resist formed from thepresently disclosed polymer displays high sensitivity.

The presently disclosed polymer preferably comprises at least 30 mol %and no greater than 70 mol % of methyl α-chloroacrylate units.

<Proportion of Components Having Molecular Weight of Less than 6,000>

In the presently disclosed polymer, the proportion of components havinga molecular weight of less than 6,000 is required to be no greater than0.5%, is more preferably no greater than 0.15%, and is even morepreferably no greater than 0.1%. If the proportion of components havinga molecular weight of less than 6,000 is greater than 0.5%, the filmreduction rate under low irradiation during use as a positive resistcannot be adequately reduced. On the other hand, when the proportion ofcomponents having a molecular weight of less than 6,000 is no greaterthan 0.15%, in addition to further reducing the film reduction rateunder low irradiation, the γ value during use as a positive resist canbe increased.

The γ value expresses the magnitude of the gradient of a sensitivitycurve indicating a relationship between the common logarithm of theirradiation dose of ionizing radiation or the like and the remainingfilm thickness of the resist after development. By raising the γ value,a pattern having higher resolution can be obtained.

<Proportion of Components Having Molecular Weight of Less than 10,000>

In the presently disclosed polymer, the proportion of components havinga molecular weight of less than 10,000 is preferably no greater than0.5%, and more preferably no greater than 0.3%. When the proportion ofcomponents having a molecular weight of less than 10,000 is no greaterthan 0.5%, the film reduction rate under low irradiation during use as apositive resist can be further reduced.

<Proportion of Components Having Molecular Weight of Greater than80,000>

In the presently disclosed polymer, the proportion of components havinga molecular weight of greater than 80,000 is preferably at least 15%,and more preferably at least 20%, and is preferably no greater than 40%.When the proportion of components having a molecular weight of greaterthan 80,000 is at least 15%, the film reduction rate under lowirradiation can be further reduced, and when the proportion ofcomponents having a molecular weight of greater than 80,000 is nogreater than 40%, the polymer can be favorably used as a positive resisthaving excellent characteristics in terms of sensitivity and so forth.

<Proportion of Components Having Molecular Weight of Greater than100,000>

In the presently disclosed polymer, the proportion of components havinga molecular weight of greater than 100,000 is preferably at least 7%,and more preferably at least 10%, and is preferably no greater than 30%,and more preferably no greater than 25%. When the proportion ofcomponents having a molecular weight of greater than 100,000 is at least7%, the film reduction rate under low irradiation can be furtherreduced, and when the proportion of components having a molecular weightof greater than 100,000 is no greater than 30%, the polymer can befavorably used as a positive resist having excellent characteristics interms of sensitivity and so forth.

The “proportion of components having a molecular weight of greater than100,000” referred to herein can be determined using a chromatogramobtained through gel permeation chromatography by calculating the totalarea (E) of peaks in the chromatogram for components having a molecularweight of greater than 100,000 as a proportion (=(E/A)×100%) relative tothe total area (A) of all peaks in the chromatogram.

<Weight Average Molecular Weight>

The weight average molecular weight (Mw) of the presently disclosedpolymer is preferably at least 36,000, more preferably at least 55,000,and even more preferably at least 60,000, and is preferably no greaterthan 100,000, and more preferably no greater than 70,000. When theweight average molecular weight (Mw) of the polymer is at least 36,000,the film reduction rate under low irradiation can be further reduced,and when the weight average molecular weight (Mw) of the polymer is nogreater than 100,000, the polymer can be favorably used as a positiveresist having excellent characteristics in terms of sensitivity and soforth.

<Number Average Molecular Weight>

The number average molecular weight (Mn) of the presently disclosedpolymer is preferably at least 25,000, more preferably at least 30,000,and even more preferably at least 42,000, and is preferably no greaterthan 100,000, and more preferably no greater than 50,000. When thenumber average molecular weight (Mn) of the polymer is at least 25,000,the film reduction rate under low irradiation can be further reduced,and when the number average molecular weight (Mn) of the polymer is nogreater than 100,000, the polymer can be favorably used as a positiveresist having excellent characteristics in terms of sensitivity and soforth.

The “number average molecular weight (Mn)” referred to herein can bemeasured by gel permeation chromatography.

<Molecular Weight Distribution>

The molecular weight distribution (Mw/Mn) of the presently disclosedpolymer is preferably less than 1.48, and more preferably no greaterthan 1.47, and is preferably at least 1.20, more preferably at least1.24, and even more preferably at least 1.27. When the molecular weightdistribution (Mw/Mn) of the polymer is less than 1.48, the polymer canbe favorably used as a positive resist having excellent characteristicsin terms of γ value and so forth, and when the molecular weightdistribution (Mw/Mn) of the polymer is at least 1.20, production of thepolymer is simple.

The “molecular weight distribution (Mw/Mn)” referred to herein is theratio of the weight average molecular weight (Mw) relative to the numberaverage molecular weight (Mn).

(Production Method of Polymer)

The polymer having the properties set forth above can be produced by,for example, polymerizing a monomer composition containingα-methylstyrene and methyl α-chloroacrylate, and then purifying theresultant polymerized product.

The composition, molecular weight distribution, weight average molecularweight, and number average molecular weight of the polymer, and theproportions of components having various molecular weights in thepolymer can be adjusted by altering the polymerization conditions andthe purification conditions. In one specific example, the weight averagemolecular weight and the number average molecular weight can be reducedby raising the polymerization temperature. In another specific example,the weight average molecular weight and the number average molecularweight can be reduced by shortening the polymerization time.

<Polymerization of Monomer Composition>

The monomer composition used in production of the presently disclosedpolymer may be a mixture containing monomers (inclusive ofα-methylstyrene and methyl α-chloroacrylate), a solvent, apolymerization initiator, and optionally added additives. Polymerizationof the monomer composition may be carried out by a known method.Cyclopentanone or the like is preferably used as the solvent and aradical polymerization initiator such as azobisisobutyronitrile ispreferably used as the polymerization initiator.

The composition of the polymer can be adjusted by altering thepercentage content of each monomer in the monomer composition used inpolymerization. The proportion of high-molecular weight componentscontained in the polymer can be adjusted by altering the amount of thepolymerization initiator. For example, the proportion of high-molecularweight components can be increased by reducing the amount of thepolymerization initiator.

A polymerized product obtained through polymerization of the monomercomposition may be collected by adding a good solvent such astetrahydrofuran to a solution containing the polymerized product andsubsequently dripping the solution to which the good solvent has beenadded into a poor solvent such as methanol to coagulate the polymerizedproduct, and the polymerized product may then be purified as describedbelow. However, the polymerized product is not specifically limited tobeing collected and purified in this manner.

<Purification of Polymerized Product>

The purification method used to purify the resultant polymerized productto obtain the polymer having the properties set forth above may be, butis not specifically limited to, a known purification method such asre-precipitation or column chromatography. Of these purificationmethods, purification by re-precipitation is preferable.

Also note that purification of the polymerized product may be repeatedmultiple times.

Purification of the polymerized product by re-precipitation is, forexample, preferably carried out by dissolving the resultant polymerizedproduct in a good solvent such as tetrahydrofuran, and subsequentlydripping the resultant solution into a mixed solvent of a good solvent,such as tetrahydrofuran, and a poor solvent, such as methanol, toprecipitate a portion of the polymerized product. When purification ofthe polymerized product is carried out by dripping a solution of thepolymerized product into a mixed solvent of a good solvent and a poorsolvent as described above, the molecular weight distribution, weightaverage molecular weight, and number average molecular weight of theresultant polymer and the proportion of low-molecular weight componentsin the resultant polymer can be easily adjusted by altering the typesand/or mixing ratio of the good solvent and the poor solvent. In onespecific example, the molecular weight of polymer that precipitates inthe mixed solvent can be increased by increasing the proportion of thegood solvent in the mixed solvent.

Also note that in a situation in which the polymerized product ispurified by re-precipitation, polymer that precipitates in the mixedsolvent of the good solvent and the poor solvent may be used as thepresently disclosed polymer, or polymer that does not precipitate in themixed solvent (i.e., polymer dissolved in the mixed solvent) may be usedas the presently disclosed polymer, so long as the polymer that is usedhas the desired properties. Polymer that does not precipitate in themixed solvent can be collected from the mixed solvent by a knowntechnique such as concentration to dryness.

(Positive Resist Composition)

The presently disclosed positive resist composition contains the polymerdescribed above and a solvent, and optionally further contains knownadditives that can be compounded in a resist composition. As a result ofthe presently disclosed positive resist composition containing theabove-described polymer as a positive resist, a high-resolution patterncan be favorably formed using a resist film obtained through applicationand drying of the presently disclosed positive resist composition.

<Solvent>

The solvent may be any known solvent in which the above-describedpolymer is soluble. Of such solvents, anisole is preferable from aviewpoint of obtaining a positive resist composition of appropriateviscosity and improving application properties of the positive resistcomposition.

EXAMPLES

The following provides a more specific description of the presentdisclosure based on examples. However, the present disclosure is notlimited to the following examples. In the following description, “%” and“parts” used in expressing quantities are by mass, unless otherwisespecified.

In the examples and comparative examples, the following methods wereused to measure and evaluate the weight average molecular weight, numberaverage molecular weight, and molecular weight distribution of apolymer, the proportions of components having various molecular weightsin the polymer, and the film reduction rate under low irradiation and γvalue of a positive resist formed from the polymer.

<Weight average molecular weight, number average molecular weight, andmolecular weight distribution>

The weight average molecular weight (Mw) and number average molecularweight (Mn) of an obtained polymer were measured by gel permeationchromatography, and then the molecular weight distribution (Mw/Mn) ofthe polymer was calculated.

Specifically, the weight average molecular weight (Mw) and numberaverage molecular weight (Mn) of the polymer were determined as valuesin terms of standard polystyrene using a gel permeation chromatograph(HLC-8220 produced by Tosoh Corporation) with tetrahydrofuran as adeveloping solvent. The molecular weight distribution (Mw/Mn) was thencalculated.

<Proportions of Components Having Various Molecular Weights in Polymer>

A chromatogram of the polymer was obtained using a gel permeationchromatograph (HLC-8220 produced by Tosoh Corporation) withtetrahydrofuran as a developing solvent. The total area (A) of allpeaks, the total area (B) of peaks for components having a molecularweight of less than 10,000, the total area (C) of peaks for componentshaving a molecular weight of less than 6,000, the total area (D) ofpeaks for components having a molecular weight of greater than 80,000,and the total area (E) of peaks for components having a molecular weightof greater than 100,000 were determined from the obtained chromatogram.The proportions of components having various molecular weights werecalculated using the following formulae.Proportion of components having molecular weight of less than10,000(%)=(B/A)×100Proportion of components having molecular weight of less than6,000(%)=(C/A)×100Proportion of components having molecular weight of greater than80,000(%)=(D/A)×100Proportion of components having molecular weight of greater than100,000(%)=(E/A)×100<Film Reduction Rate Under Low Irradiation>

A spin coater (MS-A150 produced by Mikasa Co., Ltd.) was used to apply apositive resist composition onto a silicon wafer of 4 inches in diametersuch as to have a thickness of 500 nm. The applied positive resistcomposition was heated for 3 minutes by a hot-plate at a temperature of180° C. to form a resist film on the silicon wafer. The initialthickness T₀ of the resultant resist film was measured using an opticalfilm thickness meter (Lambda Ace produced by Dainippon Screen Mfg. Co.,Ltd.). Next, an electron beam lithography device (ELS-5700 produced byElionix Inc.) was used to draw a pattern over the resist film with anirradiation dose of 16 μC/cm². Development treatment was carried out for1 minute at a temperature of 23° C. using amyl acetate (ZED-N50 producedby Zeon Corporation) as a resist developer and then rinsing was carriedout for 10 seconds using isopropyl alcohol. Next, the aforementionedoptical film thickness meter was used to measure the thickness T₁ of theresist film in a region in which the drawing had been performed. Thefilm reduction rate under low irradiation (0≤film reduction rate≤1.00)was calculated by the following formula and was evaluated in accordancewith the following standard. A smaller value for the film reduction rateunder low irradiation indicates that a high-resolution pattern can befavorably formed with a smaller amount of top-collapse.Film reduction rate under low irradiation={(T ₀ −T ₁)/T ₀}

A: Film reduction rate under low irradiation of no greater than 0.004

B: Film reduction rate under low irradiation of greater than 0.004 andno greater than 0.007

C: Film reduction rate under low irradiation of greater than 0.007 andno greater than 0.011

D: Film reduction rate under low irradiation of greater than 0.011

<γ Value>

A resist film was formed on a silicon wafer in the same way as inevaluation of the “film reduction rate under low irradiation”. Anelectron beam lithography device (ELS-5700 produced by Elionix Inc.) wasused to draw a plurality of patterns (dimensions: 500 μm×500 μm) overthe resist film with different electron beam irradiation doses. Theelectron beam irradiation dose was varied in a range of from 4 μC to 152μC in increments of 4 μC. Development treatment was carried out for 1minute at a temperature of 23° C. using amyl acetate (ZED-N50 producedby Zeon Corporation) as a resist developer and then rinsing was carriedout for 10 seconds using isopropyl alcohol. Next, an optical filmthickness meter (Lambda Ace produced by Dainippon Screen Mfg. Co., Ltd.)was used to measure the thickness of the resist film in regions in whichdrawing had been performed. A sensitivity curve was prepared thatindicated a relationship between the common logarithm of the totalelectron beam irradiation dose and the film retention rate of the resistfilm after development (=thickness of resist film afterdevelopment/thickness of resist film formed on silicon wafer). The γvalue was determined with respect to the obtained sensitivity curve(horizontal axis: common logarithm of total electron beam irradiationdose, vertical axis: film retention rate of resist film (0≤filmretention rate≤1.00)) by the following formula. In the followingformula, E₀ is the logarithm of the total irradiation dose obtained whenthe sensitivity curve is fitted to a quadratic function in a range froma film retention rate of 0.20 to a film retention rate of 0.80, and thena film retention rate of 0 is substituted with respect to the obtainedquadratic function (function of film retention rate and common logarithmof total irradiation dose). Also, E₁ is the logarithm of the totalirradiation dose obtained when a straight line is prepared that joinspoints on the obtained quadratic function corresponding to filmretention rates of 0 and 0.50 (linear approximation for gradient ofsensitivity curve), and then a film retention rate of 1.00 issubstituted with respect to the obtained straight line (function of filmretention rate and common logarithm of total irradiation dose). Thefollowing formula expresses the gradient of the straight line between afilm retention rate of 0 and a film retention rate of 1.00.

$\gamma = {{\log_{10}\left( \frac{E_{1}}{E_{0}} \right)}}^{- 1}$

The γ value was evaluated in accordance with the following standard. Alarger γ value indicates that the sensitivity curve has a largergradient and that a high-resolution pattern can be more favorablyformed.

A: γ Value of at least 9.5

B: γ Value of at least 8.5 and less than 9.5

C: γ Value of at least 7.5 and less than 8.5

D: γ Value of less than 7.5

Example 1

<Production of Polymer>

[Polymerization of Monomer Composition]

A monomer composition containing 3.0 g of methyl α-chloroacrylate and6.88 g of α-methylstyrene as monomers, 2.47 g of cyclopentanone as asolvent, and 0.02182 g of azobisisobutyronitrile as a polymerizationinitiator was added into a glass container. The glass container wastightly sealed and purged with nitrogen, and was then stirred for 6.5hours in a 78° C. thermostatic chamber under a nitrogen atmosphere.Thereafter, the glass container was returned to room temperature, theinside of the glass container was exposed to the atmosphere, and then 30g of tetrahydrofuran (THF) was added to the resultant solution. Thesolution to which the THF had been added was then dripped into 300 g ofmethanol to precipitate a polymerized product. Thereafter, the solutioncontaining the polymerized product that had been precipitated wasfiltered using a Kiriyama funnel to obtain a white coagulated material(polymerized product). The obtained polymerized product had a weightaverage molecular weight (Mw) of 35,000 and a molecular weightdistribution (Mw/Mn) of 1.60. Moreover, the obtained polymerized productcomprised 50 mol % of α-methylstyrene units and 50 mol % of methylα-chloroacrylate units.

[Purification of Polymerized Product]

Next, the obtained polymerized product was dissolved in 100 g of THF andthe resultant solution was then dripped into a mixed solvent comprising600 g of THF and 400 g of methanol (MeOH) to precipitate a whitecoagulated material (polymer including α-methylstyrene units and methylα-chloroacrylate units). Thereafter, the solution containing the polymerthat had been precipitated was filtered using a Kiriyama funnel toobtain a white polymer. The weight average molecular weight, numberaverage molecular weight, and molecular weight distribution of theobtained polymer, and the proportions of components having variousmolecular weights in the obtained polymer were measured. The results areshown in Table 1.

<Production of Positive Resist Composition>

The obtained polymer was dissolved in anisole used as a solvent toproduce a resist solution (positive resist composition) in which theconcentration of the polymer was 11 mass %. The film reduction rateunder low irradiation and γ value of a positive resist formed from thepolymer were evaluated. The results are shown in Table 1.

Example 2

A polymerized product, a polymer, and a positive resist composition wereproduced in the same way as in Example 1 with the exception that a mixedsolvent comprising 550 g of THF and 450 g of MeOH was used as the mixedsolvent in purification of the polymerized product. Measurement andevaluation were carried out in the same way as in Example 1. The resultsare shown in Table 1.

Example 3

A polymerized product, a polymer, and a positive resist composition wereproduced in the same way as in Example 1 with the exception that theamount of azobisisobutyronitrile used as a polymerization initiator inpolymerization of the monomer composition was changed to 0.03273 g and amixed solvent comprising 550 g of THF and 450 g of MeOH was used as themixed solvent in purification of the polymerized product. Measurementand evaluation were carried out in the same way as in Example 1. Theresults are shown in Table 1.

The pre-purification polymerized product had a weight average molecularweight (Mw) of 29,000 and a molecular weight distribution (Mw/Mn) of1.56.

Example 4

A polymerized product, a polymer, and a positive resist composition wereproduced in the same way as in Example 1 with the exception that theamount of azobisisobutyronitrile used as a polymerization initiator inpolymerization of the monomer composition was changed to 0.04364 g and amixed solvent comprising 550 g of THF and 450 g of MeOH was used as themixed solvent in purification of the polymerized product. Measurementand evaluation were carried out in the same way as in Example 1. Theresults are shown in Table 1.

The pre-purification polymerized product had a weight average molecularweight (Mw) of 24,000 and a molecular weight distribution (Mw/Mn) of1.53.

Example 5

A polymerized product, a polymer, and a positive resist composition wereproduced in the same way as in Example 1 with the exception that theamount of azobisisobutyronitrile used as a polymerization initiator inpolymerization of the monomer composition was changed to 0.01091 g.Measurement and evaluation were carried out in the same way as inExample 1. The results are shown in Table 1.

The pre-purification polymerized product had a weight average molecularweight (Mw) of 55,000 and a molecular weight distribution (Mw/Mn) of1.85.

Example 6

<Production of Polymer>

[Polymerization of Monomer Composition]

A monomer composition was polymerized to obtain a polymerized product inthe same way as in Example 1 with the exception that the amount ofazobisisobutyronitrile used as a polymerization initiator was changed to0.01091 g. The polymerized product had a weight average molecular weight(Mw) of 55,000 and a molecular weight distribution (Mw/Mn) of 1.85.

[Purification of Polymerized Product]

The obtained polymerized product was dissolved in 100 g of THF and theresultant solution was then dripped into a mixed solvent comprising 600g of THF and 400 g of MeOH to precipitate a white coagulated material.Thereafter, the solution containing the coagulated material was filteredusing a Kiriyama funnel to obtain the white coagulated material that hadprecipitated. The obtained coagulated material had a weight averagemolecular weight (Mw) of 65,000 and a molecular weight distribution(Mw/Mn) of 1.47.

Next, the obtained coagulated material was re-dissolved in 100 g of THFand the resultant solution was once again dripped into a mixed solventcomprising 650 g of THF and 350 g of MeOH to re-precipitate a whitecoagulated material. Thereafter, the solution containing the coagulatedmaterial that had been re-precipitated was filtered using a Kiriyamafunnel, and the filtrate was collected. The filtrate was concentrated todryness to obtain a white coagulated material (polymer includingα-methylstyrene units and methyl α-chloroacrylate units). The weightaverage molecular weight, number average molecular weight, and molecularweight distribution of the obtained polymer, and the proportions ofcomponents having various molecular weights in the obtained polymer weremeasured in the same way as in Example 1. The results are shown in Table1.

<Production of Positive Resist Composition>

A positive resist composition was produced in the same way as in Example1 with the exception that the polymer produced as set forth above wasused. Evaluation was carried out in the same way as in Example 1. Theresults are shown in Table 1.

Comparative Example 1

A polymerized product (polymer including α-methylstyrene units andmethyl α-chloroacrylate units) and a positive resist composition wereproduced in the same way as in Example 1 with the exception that theamount of azobisisobutyronitrile used as a polymerization initiator inpolymerization of the monomer composition was changed to 0.01091 g, andthe positive resist composition was produced using, as a polymer, apolymerized product collected by filtration after polymerization of themonomer composition, without carrying out purification of thepolymerized product. Measurement and evaluation were carried out in thesame way as in Example 1. The results are shown in Table 1.

Comparative Example 2

<Production of Polymer>

[Polymerization of Monomer Composition]

A monomer composition was polymerized to obtain a polymerized product inthe same way as in Example 1 with the exception that the amount ofazobisisobutyronitrile used as a polymerization initiator was changed to0.01091 g. The polymerized product had a weight average molecular weight(Mw) of 55,000 and a molecular weight distribution (Mw/Mn) of 1.85.

[Purification of Polymerized Product]

The obtained polymerized product was dissolved in 100 g of THF and theresultant solution was then dripped into a mixed solvent comprising 600g of THF and 400 g of MeOH to precipitate a white coagulated material.Thereafter, the solution containing the coagulated material was filteredusing a Kiriyama funnel, and the filtrate was collected. The filtratewas concentrated to dryness to obtain a white coagulated material(polymer including α-methylstyrene units and methyl α-chloroacrylateunits). The weight average molecular weight, number average molecularweight, and molecular weight distribution of the obtained polymer, andthe proportions of components having various molecular weights in theobtained polymer were measured in the same way as in Example 1. Theresults are shown in Table 1.

<Production of Positive Resist Composition>

A positive resist composition was produced in the same way as in Example1 with the exception that the polymer produced as set forth above wasused. Evaluation was carried out in the same way as in Example 1. Theresults are shown in Table 1.

TABLE 1 Compa- Compa- rative rative Example Example Example ExampleExample Example Example Example 1 2 3 4 5 6 1 2 Polymer Proportion ofcomponents 0.21 0.00 0.05 0.37 0.00 0.10 1.60 13.41 having molecularweight of less than 6,000 [%] Proportion of components 0.32 0.21 0.520.87 0.00 0.29 4.60 34.54 having molecular weight of less than 10,000[%] Proportion of components 12.44 6.52 3.20 1.84 35.18 20.16 22.89 1.28having molecular weight of greater than 80,000 [%] Proportion ofcomponents 6.05 3.00 1.23 0.68 24.25 11.53 15.67 0.82 having molecularweight of greater than 100,000 [%] Weight average molecular 5.2 × 10⁴4.1 × 10⁴ 3.6 × 10⁴ 3.3 × 10⁴ 6.5 × 10⁴ 6.4 × 10⁴ 5.5 × 10⁴ 1.7 × 10⁴weight (Mw) [-] Number average molecular 4.1 × 10⁴ 3.1 × 10⁴ 2.8 × 10⁴2.6 × 10⁴ 4.4 × 10⁴ 4.9 × 10⁴ 2.9 × 10⁴ 1.1 × 10⁴ weight (Mn) [-]Molecular weight 1.27 1.31 1.27 1.24 1.47 1.30 1.85 1.63 distribution(Mw/Mn) [-] Evaluation Film reduction rate B B C C A A D D under lowirradiation [-] γ Value [-] B A A C A A D D

It can be seen from Table 1 that in the case of positive resists formedfrom the polymers in Examples 1 to 6 in which the proportion ofcomponents having a molecular weight of less than 6,000 was no greaterthan 0.5%, a low film reduction rate under low irradiation was achievedcompared to in the case of positive resists formed from the polymers inComparative Examples 1 and 2 in which the proportion of componentshaving a molecular weight of less than 6,000 was greater than 0.5%.

INDUSTRIAL APPLICABILITY

Through the presently disclosed polymer, it is possible to provide apositive resist that has a low film reduction rate in a state of lowirradiation with ionizing radiation or the like.

Moreover, through the presently disclosed positive resist composition,it is possible to favorably form a high-resolution pattern.

The invention claimed is:
 1. A polymer comprising an α-methylstyreneunit and a methyl α-chloroacrylate unit, wherein a proportion ofcomponents having a molecular weight of less than 6,000 is no greaterthan
 0. 5%, a proportion of components having a molecular weight of lessthan 10,000 is no greater than 0.5%, the polymer has a molecular weightdistribution (Mw/Mn) of less than 1.48, and the polymer has a weightaverage molecular weight (Mw) of no greater than 70,000.
 2. The polymeraccording to claim 1, wherein a proportion of components having amolecular weight of greater than 80,000 is at least 15%.
 3. The polymeraccording to claim 1, having a weight average molecular weight (Mw) ofat least 55,000.
 4. A positive resist composition comprising the polymeraccording to claim 1 and a solvent.
 5. The polymer according to claim 1,wherein a proportion of components having a molecular weight of greaterthan 100,000 is at least 7%.